Summary of Work: We are examining the relationship between the structures of DNA polymerases and their functions, including fidelity. Accomplishments this year include the following. We provided compelling evidence that the fidelity of Family Y DNA polymerases that perform translesion DNA synthesis (TLS) is very low. This suggests a relaxed requirement for correct base pairing geometry and implies that the functions of translesion synthesis polymerases may be tightly controlled to prevent potentially mutagenic DNA synthesis in human cells. In support of these ideas, the X ray crystal structures of two of the translesion synthesis polymerases reveals an open active site that likely imposes less geometric selectivity for correct base pairing than DNA polymerases in all other families. We provided evidence that nucleotide misalignment in the active sites of TLS polymerases can generate frameshift mutations. We discovered that the Family X enzyme DNA polymerase lambda has 5'-dRPase activity and several other properties suggesting its participation in base excsion repair of DNA damage resulting from environmental stress. We proposed and tested a model for altered interactions of the polymerases active site with the DNA minor groove that explains the unusual base substitution error specificity of a Family A DNA polymerase with an amino acid replacement in the binding pocket for the nascent base pair.